The present invention relates to a method for forming bumps on a semiconductor chip or printed circuit board (PCB). More particularly, the present invention relates to a method for forming multi-layer connectors for flip chip bonding using metal powders and an adhesive or a flux.
A typical flip chip assembly uses a direct electrical connection of a face-down semiconductor chip onto a substrate or circuit board via conductive bumps. Generally, a flip chip assembly is made in three stages, forming bumps on a chip, attaching the bumped chip to a board or substrate, and filling the space remaining under the bumped chip with an electrically non-conductive material.
The conductive bump has several functions in a flip chip assembly, such as providing an electrical connection between a semiconductor chip and a substrate, providing a thermal path to carry heat from the semiconductor chip to the substrate, and providing part of the mechanical mounting to the substrate. The bump also acts as a spacer for preventing electrical contact between the semiconductor chip and substrate conductors.
There are many conventional methods of forming such conductive bumps. One method includes forming a photoresist layer having openings aligned with bond pads on a wafer, applying a solder paste in the openings by screen printing, and then melting or reflowing the solder paste to form a bump. The openings may be formed by radiating and developing the photoresist.
One problem with this method is that a new photoresist layer is required for processing each wafer. Another problem is that the photoresist layer is removed with chemical solutions, which generates chemical wastes. Yet another problem is that bump standoff (bump height) depends on the thickness of the photoresist mask. To obtain a higher standoff, a thicker photoresist layer is required.
Unfortunately, if a low or fine pitch (bump spacing) is required, the maximum possible thickness of the photoresist layer is limited. In practice, the openings in the photoresist layer typically have a reverse conical shape, i.e., the openings taper towards a narrow end at the bond pads. Hence, there is a tradeoff between a high standoff and a low pitch.
Another method of forming bumps involves patterning a photoresist layer applied to a wafer to form bump sites and electroplating a solder alloy onto the bump sites. The photoresist layer is then removed before reflowing the solder alloy to form a sphere. While this electroplating method provides a low pitch, wet chemicals or plating bath solutions are required. Further, such chemical processes involve hazardous materials that must be carefully controlled.
In view of the foregoing, it would be desirable to have a method for forming bumps that is low cost and does not involve wet chemicals. In addition, it would be desirable to have a method that provides high standoff (bump height) and low or fine pitch.